1. Field of the Invention
This invention relates generally to communication systems having characteristics of privacy and resistance to various interferences such as noise or willful or accidental jamming. More particularly, this invention relates to communication systems in which the carrier has its center frequency moved (or swept) continuously over a frequency range which is much wider than the intelligence (or data) bandwidth. This invention relates, even more particularly, to a receiver capable of tracking the center frequency of the received swept carrier by continuous self-synchronization of the tracking means so as to provide for narrowband filtering of the widely swept carrier.
2. Description of the Prior Art
Frequency-tracking receivers have been known in the art. In one embodiment, the receiver has an internal oscillator with an output which is of a different center frequency than the received carrier but of similar sweep. The sweep of the internal oscillator is of identical frequency and phase and of nearly the same amplitude as the received carrier. As the received carrier and the output of internal oscillator are mixed, an IF carrier is obtained which has only a small sweep associated with it. The result is that the IF carrier may be filtered through a narroww bandwidth.
In another embodiment, the received carrier is passed through the narrowband filter whose center frequency tracks the center frequency of the received carrier. Here again, the received carrier of a wide frequency sweep is filtered through a narrow bandwidth.
Generally speaking, frequency tracking receivers are used in two types of applications:
(1) In the first type of application, it is desired to track all deviations of the carrier from its center frequency. Thus, for example, a carrier of a complex non-predescribed deviation will have its total deviation tracked at each instant. Applications of this type include FM demodulation at a lowered noise threshold, and a reduction of the effect of incidental FM in AM transmissions.
(2) In the second type of application, it is desired to track a certain periodic component of the deviation which is known in priori in amplitude and shape, and almost known in rate and phase; all other components of the frequency deviation should not be tracked. Applications of this type are in communication systems utilizing the wideband sweeping of the carrier for privacy or resistance to jamming and noise.
The implementations of the receivers for the two types of tracking applications differ in basic ways and the present invention relates to the second type. The present invention utilizes a tracking receiver which is uniquely kept in synchronism in frequency and in phase with the received carrier. In the prior art, this synchronization has typically been achieved either by means of separately transmitted synchronization pulses as proposed by Kendall in U.S. Pat. No. 1,592,940, or by means of synchronization pulses produced in an AM manner in the receiver as described by Silver, et al, in U.S. Pat. No. 2,448,055. Thus, in the prior art, synchronism is maintained in a discrete manner via pulses. Furthermore, the synchronizing mechanism of prior art devices have not taken advantage of the noise and jamming resistance of a widely swept FM carrier.